1. Field of the Invention
The invention provides Mycobacterium strains with improved vaccinal properties for use as vaccinating agents against tuberculosis. The Mycobacterium strains are preferably selected from parent strains that are identified as halving potent immunogenicity, do not display antibiotic resistance, and do not exhibit horizontal transfer to gram-negative bacteria. The invention also provides Mycobacterium with improved properties for delivering transgenes that will have vaccinal properties for use in vaccinating against other diseases and for use in the treatment of cancer.
2. Background
Mycobacterium tuberculosis (M. tb) has infected one-third of the world's population, causing active disease in 8 million and killing 1.6-2.2 million individuals every year, most of whom live in the developing world. Tuberculosis (TB) is an epidemic of global proportions that is growing and becoming even more deadly as it intersects with the spread of HIV. TB is the number one killer of people with AIDS.
BCG, the current widely used TB vaccine, was developed over 80 years ago and when tested has had widely variable rates of efficacy against pulmonary tuberculosis, including no efficacy in the last large field trial conducted in India (Fine et al., Vaccine, 16(20):1923-1928; 1998; Anonymous, Indian J Med. Res., August; 110:56-69; 1999. Nonetheless, the World Health Organization currently recommends BCG at birth or first contact with health services for all children (except those with symptoms of HIV disease/AIDS) in high TB prevalent countries. This policy is based on evidence that BCG protects against serious childhood forms of TB (Lanckriet et al., Int J Epidemiol, 24(5): 1042-1049; 1995; Rodrigues et al., J Epidemiol Community Health 45(1): 78-80; 1991. Protection by BCG against TB beyond early childhood is a controversial subject with limited data giving mixed results. The high incidence of pediatric and adult TB in developing countries where infant BCG immunization is widely practiced, however, indicates that BCG as currently administered is not highly efficacious over the many years when people are at risk of TB disease. Thus, BCG is considered to be an inadequate public health tool for the intervention and control of TB.
Approximately 70 percent of humans exposed to TB organisms, and who have normal immune systems, do not become infected, and of those that do become infected only about 5 percent develop disease within the first two years. The majority of infected individuals suppress the infection, which is associated with the development of robust cellular immune responses to M. tb antigens. An additional 5 percent later reactivate when immunity declines. Both primary and reactivation disease are much more common in people with HIV/AIDS, again emphasizing the role of immunity in preventing and controlling infection.